Terahertz Vibrational Spectroscopy Research Articles

Investigation into polymorphism within ethenzamide-ethylmalonic acid cocrystal using Raman and terahertz vibrational spectroscopy

Two cocrystal polymorphs of ethenzamide (ETZ) and ethylmalonic acid (EMA) were synthesized by solvent evaporation. Crystal structure analysis revealed that the main amide − carboxyl heterosynthon in ETZ-EMA cocrystal Form I and Form II are the same, but the crystal structure of these two polymorphs are different. Terahertz (THz) and Raman vibrational spectroscopy were used to characterize ETZ, EMA, ETZ-EMA cocrystal polymorph Form I and Form II respectively. The experimental results showed that ETZ, EMA, ETZ-EMA cocrystal Form I and ETZ-EMA cocrystal Form II exhibited completely different characteristic peaks. Both THz and Raman vibrational spectroscopy can be used to distinguish ETZ-EMA cocrystal Form I from Form II. Furthermore, the investigation of phase transition induced by temperature and solid-state grinding was also performed. In the temperature phase transition experiments, when the powder sample was heated to a temperature range of 80–82 °C, the metastable ETZ-EMA cocrystal Form I transformed into the more stable ETZ-EMA cocrystal Form II. Solid-state grinding analysis revealed that the results of the ETZ-EMA cocrystal polymorph synthesis in grinding experiments depended on the polarity of the solvents used. Grinding without solvent or with high polarity solvents tended to result in the stable ETZ-EMA cocrystal Form II. Moreover, the metastable ETZ-EMA cocrystal Form I would transform into Form II after further grinding process. These results demonstrate that THz and Raman vibrational spectroscopy have high sensitivity and accuracy in the detection of both cocrystal synthesis and cocrystal polymorph phase transitions.

Investigating the function and design of molecular materials through terahertz vibrational spectroscopy.

Terahertz spectroscopy has proved to be an essential tool for the study of condensed phase materials. Terahertz spectroscopy probes the low-frequency vibrational dynamics of atoms and molecules, usually in the condensed phase. These nuclear dynamics, which typically involve displacements of entire molecules, have been linked to bulk phenomena ranging from phase transformations to semiconducting efficiency. The terahertz region of the electromagnetic spectrum has historically been referred to as the 'terahertz gap', but this is a misnomer, as there exist a multitude of methods for accessing terahertz frequencies, and now there are cost-effective instruments that have made terahertz studies much more user-friendly. This Review highlights some of the most exciting applications of terahertz vibrational spectroscopy so far, and provides an in-depth overview of the methods of this technique and its utility to the study of the chemical sciences.

Nanoconfinement of tetraphenylethylene in zeolitic metal-organic framework for turn-on mechanofluorochromic stress sensing

Mechanofluorochromic materials are of great significance for the fabrication of innovative sensors and optoelectronics. However, efficient mechanofluorochromic materials are rarely explored due to the deficiency of existing design strategies. Here, we demonstrate the incarceration of aggregation-induced emission (AIE) materials within metal-organic framework (MOF) single crystals to construct a composite system with turn-on mechanofluorochromism. A new type of AIE@MOF material was designed: integrating a zeolitic MOF (ZIF-71) and tetraphenylethylene (TPE, a topical AIE material) to generate a TPE@ZIF-71 system with exceptional turn-on type mechanofluorochromism. Using terahertz vibrational spectroscopy, we show the unique fluorochromism mainly emanates from the enhanced nanoconfinement effect exerted by ZIF-71 host on TPE guest under pressure. Compared with pure TPE, we demonstrate the nanoconfinement in AIE@MOF not only changes the TPE's turn-off type sensing behavior to a turn-on type, but boosts the original sensitivity markedly by tenfold. Significantly, because ZIF-71 prevents the spontaneous recrystallization of TPE upon unloading, this allows TPE@ZIF-71 to record the stress history. This is the first demonstration of the Guest@MOF system combining the concepts of AIE and MOF; its promising properties and potential engineering applications will stimulate new directions pertaining to luminescent stress sensors and smart optics.

Dynamic Detection of Thiol Oxidation/Reduction Status During the Conversion of Cysteine/Cystine

• Dynamic detection of thiol oxidation/reduction status during the conversion of cysteine/cystine. • The thiol redox state is intuitively reflected by terahertz spectroscopy and Raman spectroscopy in varying degrees. The thiol redox state (TRS) plays a key role in evaluating essential biochemical processes, such as evaluating intracellular enzymatic activity. The TRS, characterized by the kinetic parameters of the thiol (–SH)/disulfide bond (–SS–) redox process, was measured by the Fourier-transform Raman spectroscopy (FT–Raman) and the terahertz time-domain spectroscopy (THz–TDS) in this study. A good correlation was observed between the disulfide concentration in samples and the intensities of the disulfide characteristic peaks in the FT–Raman and THz–TDS spectra. And the linear regression analysis correlation coefficients had values of 92.2% for FT–Raman-derived data and 95.4% for THz–TDS-derived data. The dynamic response of the cysteine/cystine experimental spectrum and characteristic peaks were analyzed by solid-state density functional theory. It can be found that both Raman and terahertz vibrational spectroscopy techniques can quantitatively analyze the ratio of cysteine/cystine. This provides a new method for the biological analysis of sulfur-containing amino acids and other related mixtures in basic science.

The necessity of periodic boundary conditions for the accurate calculation of crystalline terahertz spectra.

Terahertz vibrational spectroscopy has emerged as a powerful spectroscopic technique, providing valuable information regarding long-range interactions - and associated collective dynamics - occurring in solids. However, the terahertz sciences are relatively nascent, and there have been significant advances over the last several decades that have profoundly influenced the interpretation and assignment of experimental terahertz spectra. Specifically, because there do not exist any functional group or material-specific terahertz transitions, it is not possible to interpret experimental spectra without additional analysis, specifically, computational simulations. Over the years simulations utilizing periodic boundary conditions have proven to be most successful for reproducing experimental terahertz dynamics, due to the ability of the calculations to accurately take long-range forces into account. On the other hand, there are numerous reports in the literature that utilize gas phase cluster geometries, to varying levels of apparent success. This perspective will provide a concise introduction into the terahertz sciences, specifically terahertz spectroscopy, followed by an evaluation of gas phase and periodic simulations for the assignment of crystalline terahertz spectra, highlighting potential pitfalls and good practice for future endeavors.

Simulation and Assignment of the Terahertz Vibrational Spectra of Enalapril Maleate Cocrystal Polymorphs.

The identification of crystalline drug polymorphs using terahertz vibrational spectroscopy is a powerful approach for the nondestructive and noninvasive characterization of solid-state pharmaceuticals. However, a complete understanding of the terahertz spectra of molecular solids is challenging to obtain because of the complex nature of the low-frequency vibrational motions found in the sub-3 THz (sub-100 cm-1) range. Unambiguous assignments of the observed spectral features can be achieved through quantum mechanical solid-state simulations of crystal structures and lattice vibrations utilizing the periodic boundary condition approach. The terahertz spectra of two polymorphs of enalapril maleate are presented here to demonstrate that even large pharmaceuticals can be successfully modeled using solid-state density functional theory, including cocrystalline solids comprised of multiple distinct species. These simulations enable spectral assignments to be made, but also provide insights into the conformational and cohesion energies that contribute to the polymorph stabilities. The results reveal that the Form II polymorph of enalapril maleate is the more stable of the two under ambient conditions, and that this stability is driven by a greater intermolecular cohesion energy as compared to Form I.

Thermoelasticity in organic semiconductors determined with terahertz spectroscopy and quantum quasi-harmonic simulations

The thermomechanical response of organic semiconducting solids – an essential aspect to consider for the design of flexible electronics – was determined using terahertz vibrational spectroscopy and quantum quasiharmonic approximation simulations.

Distinguishing Nonlinear Terahertz Excitation Pathways with Two-Dimensional Spectroscopy.

High-field terahertz (THz) spectroscopy is enabling the ultrafast study and control of matter in new and exciting ways. However, when intense electromagnetic pulses are used in any kind of pump-probe spectroscopy, several nonlinear excitation pathways can result, leading to scenarios that required the development of multidimensional spectroscopies to illuminate the observed dynamics. Here we demonstrate a clear example where two-dimensional (2D) THz vibrational spectroscopy is needed to distinguish between nonlinear-excitation pathways in CdWO_{4}. We nonlinearly excite a set of Raman-active vibrational modes in CdWO_{4} with broadband THz pulses, and 2D spectroscopy allows us to determine the dominant excitation pathway. We provide a general framework for 2D THz and multi-THz nonlinear phonon spectroscopy in solid systems, which has important implications in contributing needed clarity to the nascent field of nonlinear phononics.

Metamaterial-enhanced terahertz vibrational spectroscopy for thin film detection

We present metamaterial-enhanced terahertz vibrational spectroscopy to solve the low sensitivity problem of the THz ray absorption detection in molecular and biomolecular thin film. In a proof-of-principle experiment, we demonstrate the system in split ring resonators (SRRs) metamaterial that is strongly coupled to L-tartaric acid molecular under a low-temperature condition. The experimental results show that the extinction ratio of the detected signal can be significantly improved from 1.75 dB to 4.5 dB. The numerical calculations confirm and explain the experimental observations. By detuning the resonance of metamaterial, the behavior of the spectral signal is modified. When the SRRs and molecular vibrational resonance frequencies are closely aligned, a clear mode splitting is observed resulting in a transparency transmission with enhanced extinction ratio. This method shows great potential for application in thin film sensing by detecting molecular vibrations in the lower-energy terahertz region.

Polymorphism in cis–trans Muconic Acid Crystals and the Role of C–H···O Hydrogen Bonds

Cis–trans isomerization of muconic acid is an important component of various chemical syntheses. Despite the existence of three geometric muconic acid isomers, little atomically precise information is actually known of their structures. In this work, the complete crystal structures of cis–cis, cis–trans, and trans–trans muconic acid were determined via single-crystal X-ray diffraction and then further analyzed using terahertz vibrational spectroscopy and solid-state density functional theory. In addition to being the first reported crystal structure for cis–trans muconic acid, two polymorphs of this species were discovered, with one exhibiting a two-dimensional hydrogen-bonding pattern stabilized through C–H···O interactions. Terahertz spectroscopy aided in polymorph characterization and also provided clear evidence of a hydrated crystalline species that grows only from one of the cis–trans muconic acid polymorphs. Ab initio simulations showed that while the typical end-to-end hydrogen-bonding motif found...

Concomitant polymorphism and the martensitic-like transformation of an organic crystal.

Crystalline polymorphism is a phenomenon that occurs in many molecular solids, resulting in a diverse range of possible bulk structures. Temperature and pressure can often be used to thermodynamically control which crystal form is preferred, and the associated transitions between polymorphic phases are often discontinuous and complete. N-Methyl-4-carboxypyridinium chloride is a solid that undergoes an apparent continuous temperature-dependent phase transition from an orthorhombic to a monoclinic polymorph. However, a hybrid characterization approach using single-crystal X-ray diffraction, terahertz time-domain spectroscopy, and solid-state density functional theory reveals the transformation to be actually a slowly changing ratio of the two discrete polymorphic forms. The potential energy surface of this process can be directly accessed using terahertz radiation, and the data show that a very low barrier (43.3 J mol-1) exists along the polymorph transformation coordinate.

Elucidation of Chiral Symmetry Breaking in a Racemic Polymer System with Terahertz Vibrational Spectroscopy and Crystal Orbital Density Functional Theory.

The conservation of chiral symmetry has been used as a fundamental rule to determine polymer packing conformations in racemic systems. We have illustrated, through the interplay of polarization terahertz (THz) spectroscopy and solid-state density functional theory, that the chiral symmetry is not conserved in a poly(lactic acid) stereocomplex (scPLA) system. Poly(l-lactic acid) (PLLA) displays a weaker violation of the 31 screw symmetry than poly(d-lactic acid) (PDLA) and possesses a stronger intramolecular vibrational energy, on average, in the low-frequency gamma-point phonon modes than does PDLA. Polarization THz spectroscopy adds a new dimension to polymer crystallography through which new phenomena are expected to be revealed.

Uncovering the Terahertz Spectrum of Copper Sulfate Pentahydrate.

Terahertz vibrational spectroscopy has evolved into a powerful tool for the detection and characterization of transition metal sulfate compounds, specifically for its ability to differentiate between various hydrated forms with high specificity. Copper(II) sulfate is one such system where multiple crystalline hydrates have had their terahertz spectra fully assigned, and the unique spectral fingerprints of the forms allows for characterization of multicomponent systems with relative ease. Yet the most commonly occurring form, copper(II) sulfate pentahydrate (CuSO4·5H2O), has proven elusive due to the presence of a broad absorption across much of the terahertz region, making the unambiguous identification of its spectral signature difficult. Here, it is shown that the sub-100 cm(-1) spectrum of CuSO4·5H2O is obscured by absorption from adsorbed water and that controlled drying reveals sharp underlying features. The crystalline composition of the samples was monitored in parallel by X-ray diffraction as a function of drying time, supporting the spectroscopic results. Finally, the terahertz spectrum of CuSO4·5H2O was fully assigned using solid-state density functional theory simulations, helping attribute the additional absorptions that appear after excessive drying to formation of CuSO4·3H2O.

Structure of soft-materials and hydrogen-bonds observed by terahertz vibrational spectroscopy

Structure of soft-materials and hydrogen-bonds observed by terahertz vibrational spectroscopy

Experimental and Theoretical Investigations on the Terahertz Vibrational Spectroscopy of Alanine Crystal

The terahertz absorption and Raman scattering spectra of an alanine crystal in the range of 0.2-2.6 THz were obtained using terahertz time-domain spectroscopy and low-frequency Raman spectroscopy. The results indicated that there were four vibrational modes in this low-frequency region. Two modes were Raman active whereas the other two were both infrared and Raman active. A theoretical investigation on the periodic structure of alanine was performed using a self-consistent field crystal orbital method based on the B3LYP hybrid density functional. By comparing the experimental and theoretical results, irreducible representations were assigned to the corresponding peaks in the spectra. It was indicated that the vibrational modes in this low-frequency region were mainly torsion or rocking modes involving inter-molecular hydrogen bonds which have been described using schematic representations.

Investigation of crystalline 2-pyridone using terahertz spectroscopy and solid-state density functional theory

Crystalline 2-pyridone has been investigated using terahertz vibrational spectroscopy in the range of 10–90 cm −1 (0.3–2.7 THz). Solid-state density functional theory (B3LYP, BP, and PW91 with the 6-311G(d,p) basis set) was used to simulate and assign both observed terahertz spectral features and a previously published far-infrared spectrum up to 400 cm −1. The PW91 functional was found to provide the best combination of crystal structure and vibrational frequency reproduction. Observed spectral features below 150 cm −1 are assigned to intermolecular movements of the 2-pyridone chains within the unit cell. The use of independent intramolecular and intermolecular frequency scalars is proposed.